Power switches and voltage regulators are just two examples of integrated circuits that employ capacitors as a key part of their operation. Depending on how their operation ceases, such capacitors may be left with a residual charge that bleeds off very slowly. Circuits that presume that such capacitors are fully discharged for subsequent operations may exhibit undesired behavior when power is restored after an unexpected interruption. As an example, low dropout regulators (LDOs) often rely on charging of an external capacitor for slowly ramping up the output voltage (“soft-start”). When the external capacitor is left with a residual charge, proper ramping is inhibited and may result in impermissible inrush current levels.
One approach to this problem relies on depletion-mode MOSFETs (metal-oxide-semiconductor field-effect transistors) whose conduction is inhibited when power is applied to the source. A loss of power returns the MOSFET to a conductive state, enabling it to discharge internal capacitances. However, at least some of the preferred semiconductor process flows do not provide for inclusion of depletion-mode MOSFETs.